Low-level service calls¶

If you need full control over a request — fill in every optional field, override a default, or capture the raw response — every service the o6 client supports has a serviceX method that takes a pre-built request object and returns a pre-built response. The high-level methods (client.read, client.write, client.call, client.browse, …) are thin wrappers over the same calls.

This page walks through the low-level basics:

• Build a request object.
• Call the corresponding serviceX method.
• Read fields off the response.

Build a request object¶

Each service has a paired request type that lives in o6.ns.ns0. The request has the same fields as the spec's request message — required fields set, optional fields left at their defaults:

from o6.ns.ns0.datatypes import ReadRequest, ReadValueId

req = ReadRequest()
req.nodesToRead = [ReadValueId("ns=1;i=1204")]

ReadValueId accepts a plain NodeId string directly in its constructor for the simple case of reading the Value attribute — you only need to build it up field-by-field (attributeId=, indexRange=, dataEncoding=) when you're after something other than Value. Request types themselves (ReadRequest, WriteRequest, …) take no constructor arguments — build one with Type() and set its fields by attribute, as above.

Call the corresponding serviceX method¶

The suffix of a serviceX method is the OPC UA service name. Pass the request, get the response back:

from o6 import Client

with Client("opc.tcp://localhost:4840") as client:
    response = client.serviceRead(req)

The complete list of serviceX methods on the client mirrors the OPC UA service sets:

• Discovery: serviceFindServers, serviceFindServersOnNetwork, serviceGetEndpoints
• Node management: serviceAddNodes, serviceDeleteNodes, serviceAddReferences, serviceDeleteReferences
• View: serviceBrowse, serviceBrowseNext, serviceTranslateBrowsePathsToNodeIds, serviceRegisterNodes, serviceUnregisterNodes
• Attribute: serviceRead, serviceHistoryRead, serviceWrite, serviceHistoryUpdate
• Method: serviceCall

Every serviceX method returns a MaybeAwaitable to be readily used in both sync and async contexts — add await in async code:

async with Client("opc.tcp://localhost:4840") as client:
    response = await client.serviceRead(req)

Read fields off the response¶

The response object mirrors the spec's response message. Read the fields you care about the same way you read any Python attribute:

with Client("opc.tcp://localhost:4840") as client:
    req = ReadRequest()
    req.nodesToRead = [ReadValueId("ns=1;i=1204")]   # Status.Setpoint
    response = client.serviceRead(req)

    # First check the service-level result code
    print(response.responseHeader.serviceResult)   # StatusCode.Good

    # Then read the per-node results
    result = response.results[0]
    print(result.status)         # StatusCode.Good
    print(result.value)          # e.g. 90.0

A non-Good responseHeader.serviceResult means the whole service call failed (e.g. the secure channel is in a bad state). A non-Good per-node status means only that node failed. The serviceX methods do not raise on either — they hand you the raw response and let you decide.

What's next?¶

• Read / write value and Call a method — the high-level wrappers.
• The Client API reference — full surface of every serviceX method.